Nowadays, lightweight, non-shattering, transparent resin materials are widely used as a substitute for transparent flat glass. For example, plastic substrates, especially polycarbonate resins, due to their excellent properties including transparency, impact resistance, and heat resistance, are currently used as structural members in place of glass, in various applications including windows in vehicles and buildings, meter covers and the like. However, it is strongly desired to improve the surface properties of plastic substrates, typically polycarbonate resins because their surface properties like mar resistance and weatherability are inferior to glass.
Known means for improving the mar resistance of molded polycarbonate resins is by coating the surface of a resin substrate with a thermosetting resin such as organopolysiloxane. Known means for improving weatherability is by covering the surface of a resin substrate with a weather-resistant resin layer containing a UV absorber. Coating compositions known to form surface protective films for the purpose of imparting high hardness and mar resistance to the resin substrate surface include a composition comprising a hydrolyzate or partial hydrolyzate of hydrolyzable organosilane and a composition comprising the hydrolyzate or partial hydrolyzate and colloidal silica as described in JP-A S63-168470. As the coating composition having excellent UV resistance to be applied to automotive glazing plastic substrates, JP-A 2011-037969 and JP-A 2012-056098 disclose a coating system comprising a hard coat coating agent comprising a hydrolyzate of UV-absorbing organoalkoxysilane and/or partial condensate thereof and colloidal silica in combination with an acrylic resin primer having improved UV resistance, and a coating method using such a combination.
One practice for coating a plastic substrate with a silicone coating agent is by flow coating a primer, then coating and curing a silicone hard coat paint of heat cure type thereto to form a coating film.
Such hard coat films have a high hardness enough to impart mar resistance to the substrate for protection, but lack toughness, allowing a multilayer thick film to readily crack in response to sharp temperature changes during heat curing or outdoor service. Another drawback is that the hard coat film will crack during long-term outdoor service because it cannot conform to changes with time of the substrate or primer layer caused by UV, acid, base, water and other factors. In such cases, where the plastic substrate has a silicone coating thereon, one prior art approach for repairing cracks or defects in a damage area of a coating is by stripping off the original coating around the damage area and applying a topcoat paint thereto again.
Also, if the coating layer in the coating step includes thickness variations or has deposits or foreign particles thereon, repair is carried out by removing the relevant coating layer portion by such means as grinding, and coating refinish paint thereon. Since it is difficult to form a silicone layer directly on the surface of a once cured silicone film in a tight bond, a primer layer capable of tight bonding must be previously formed. Thus, the process becomes cumbersome and time-consuming. In repairing a defective portion of a coating caused by cracks and coating variations, the original coating formed by flow coating and a refinish coating on the defective portion formed by manual air spraying may differ in the cured state. In such a case, the refinish coating on the defective portion may not have the luster appearance of the original coating, presenting a strange appearance.